1. Field of the Invention
The present invention is directed to joined solid oxide fuel cell stacks, and more particularly to a solid oxide fuel cell system and method of fabricating same.
2. Background Art
Generally, solid oxide fuel cells are designed for a service life of at least five years with a performance decay of less than 0.5% per 1000 hours of power output. Performance decay in fuel cell stacks has several sources. These sources include changes in electrode microstructure, delamination of electrodes from the electrolyte and cracked ceramic components (where ceramic components are utilized).
Solid oxide fuel cells are generally arranged in stacks of individual solid oxide fuel cells (electrolyte with electrodes) separated by interconnects. Interconnects provide reactant gas conduits and provide electrical series connection of the individual cells. The failure of one cell in a stack drastically affects the performance of the entire stack, which is exacerbated over time. In particular, a failed cell will exhibit higher temperature due to the direct mixing of reactant gasses or due to the higher resistance which is caused by physical or electrochemical changes in electrodes of the cell. Such an increase in temperature in the failed cell likewise detrimentally affects the adjoining cells, increasing cell degradation thereof.
Further, since the temperature is higher in one cell (i.e. the failed cell), a temperature variation or gradient appears across the stacks. Since the temperature of the cell is directly related to resistance, while all of the cells are identical, the cells in the cooler region would show a higher resistance. As a result, the cooler cells will be at a lower operating potential than the cells in the hotter region again affecting total output. Thus, the failure of even a single cell adversely affects the cell stack.
Certain solutions have been proposed to minimize the effect of a failed cell. For example, as disclosed in U.S. Pat. No. 5,238,754, multiple cells are arranged on a single large interconnect to render a cell/interconnect unit. Subsequently, the cell/interconnect units are vertically layered so as to form a stack. The purpose of such a construction is to provide for a large active area by the summation of multiple small area cells on a single interconnect. Further, since the cells of any one unit are essentially connected in parallel, the cells of a single unit provide redundant electrical paths. While this solution has had some success, there are several drawbacks. First, it is difficult to make large area interconnects that can meet the required flatness requirements. In addition, the interior edges of each cell of each unit cannot be inspected for seal effectiveness. In addition, this arrangement does not protect against seal or cell failures which will allow reactant cross-over to affect downstream cells in the frames.
Thus, it would be an object of the invention to improve reliability of the fuel cell stack by providing alternate and/or redundant electrical paths around failed cells.
It would also be an object of the invention to even out the voltage distribution among the cells in the stack.
It would further be an object of the invention to equalize the temperature distribution in the stack.
It would further be an object of the invention to minimize the thermal stresses and sealing problems of the cell due to differences in thermal expansion across the stack.
These and other objects will become apparent in light of the specification and claims.
The invention comprises a solid oxide fuel cell system. The system comprises at least two solid oxide fuel cell stacks and at least one extension member. Each solid oxide fuel cell stack includes a plurality of solid oxide fuel cells. Each cell is separated by an interconnect. The extension member joins at least one interconnect of one of the solid oxide fuel cell stacks with a corresponding interconnect of another of the solid oxide fuel cell stack.
In a preferred embodiment, the at least one extension member comprises a plurality of extension members. Each extension member joins an interconnect of one solid oxide fuel cell stack with a corresponding interconnect of another of the solid oxide fuel cell stack. In one such preferred embodiment, wherein each stack includes at least five interconnects, an extension member joins every fifth interconnect of one solid oxide fuel cell stack with a corresponding interconnect of another solid oxide fuel cell stack.
In another preferred embodiment, the system includes three solid oxide fuel cell stacks.
In such an embodiment, the extension member joins one interconnect of one solid oxide fuel cell stack to an interconnect of each of the other solid oxide fuel cell stacks. In one embodiment, the extension member includes a hub and at least one spoke extending from the hub to each corresponding interconnect of each solid oxide fuel cell stack.
In another preferred embodiment, the system includes four solid oxide fuel cell stacks. In one such embodiment, the extension member joins an interconnect of one of the fuel cell stacks to the corresponding interconnect of each of the remaining fuel cell stacks. In one such embodiment, the extension member may comprise a hub and a spoke extending from the hub to each corresponding interconnect of each solid oxide fuel cell stack. In another such embodiment, the extension member may individually connect each corresponding interconnect of each fuel cell stack.
In another preferred embodiment, the extension member is configured so as to facilitate access to substantially the entirety of the outer perimeter of the fuel cells of each of the fuel cell stacks.
In yet another embodiment, the system further includes at least one current collector associated with the at least one extension member.
The invention further includes a method of fabricating a solid oxide fuel cell system. The method comprises the steps of providing at least two solid oxide fuel cells stacks and associating at least one interconnect of one solid oxide fuel cell stack to a corresponding interconnect of another of the solid oxide fuel cell stacks by way of the extension member.
In a preferred embodiment, the step of attaching comprises the step of associating at least one interconnect of one solid oxide fuel cell stack to a corresponding interconnect of each of the remaining solid oxide fuel cell stacks by way of an extension member.
In another embodiment, the step of attaching comprises the step of repeating the step of attaching for each of a predetermined plurality of extension members.
In another preferred embodiment, the step of associating a current collector with the extension member.